Add "sync with upstream" functionality for GitHub [SG-10470]

Should be easy enough to add: I'd like to be able to sync my fork (origin) with the original repo (upstream). It's great that smartgit already suggests the creation of the upstream remote, now it'd be even greater if there was a "one-click" (I know ...) solution for syncing (available from the "Branches" view?) origin and upstream. This would be (according to GitHub's help):

git fetch <upstream>
git checkout <branch>
git merge <upstream>/<branch>